The strength of glass articles can be improved in that the tensile stresses in the surface are reduced. These tensile stresses become a problem with respect to the glass. This reduction is achieved in that the glass surface is subjected to compressive stresses. This operation is often characterized as tempering. A fracture occurs only when the load has reached at least the magnitude of the compressive stresses.
In addition to thermal tempering (that is, quenching of glass articles) different methods of chemical tempering are known. They are based on ion exchange processes. Methods are known which are carried out above the transformation point Tg of the particular glass as well as methods which provide for working below the transformation point Tg. The first-mentioned methods have the disadvantage that a deformation of the glass article can occur above the transformation point Tg.
Compressive stresses can be generated below the transformation point Tg in practically rigid glass networks utilizing an ion exchange. To achieve this, existing ions having a small diameter must be replaced at the surface with ions having a larger diameter.
Alkali ions are suitable for this exchange because of their good mobility. For example, and for a soda-lime glass in a potassium salt bath, Na.sup.+ ions migrate out of the glass into the salt bath and K.sup.+ ions migrate from the bath into the glass. This is then a diffusion process. A diffusing ion overcomes a potential barrier proportional to the charge. For this reason, and as set forth in the article of Schroder et al entitled "Festigkeitserhohung von Glasern durch Oberflachenbehandlung", Naturwissenschaften, 57, pages 533 to 541 (1970), it is not to be expected that ions having a dipositive charge and a radius, which is small relative to the alkalis, are capable of migration at the moderately high temperatures (which are adequate for the diffusion of alkali ions and which temperatures are required for chemical tempering based on the described significance of the transformation point Tg for these methods). Therefore, the technical and patent literature focusses on methods for exchanging alkali ions (Li.sup.+, Na.sup.+) from the glass out or alkali ions (Na.sup.+, K.sup.+) or other univalent ions (Ag.sup.+, Au.sup.+) into the glass.
A glass suitable for chemical pretensioning requires a sufficient content of ions capable of exchange in accordance with accepted teaching, that is, a sufficient content of alkali ions.
The temperature difference stability of glass articles, which is required for many applications, is now improved, contrary to accepted teaching, by the reduction of the alkali content in the glass. The temperature difference stability characterizes the capability of, for example, a glass plate to withstand the temperature difference between the hot center and the cold edge of the plate. A well known example is the group of borosilicate glasses which exhibit an excellent temperature change stability and temperature difference strength because of their low thermal coefficient of expansion .alpha..
Accordingly, borosilicate glasses have been viewed for decades as not being chemically temperable and this view has been strengthened by unsuccessful experiments of chemical pretensioning of one of the best known examples of borosilicate glasses, namely, borosilicate glass 3.3 (composition in percent by weight on oxide basis of: SiO.sub.2 80.9; B.sub.2 O.sub.3 12.8; Al.sub.2 O.sub.3 2.4; Na.sub.2 O 3.3; K.sub.2 O 0.6). In this connection, reference can again be made to the above-mentioned article of Schroder et al.
In PCT patent publication WO 96/01792, a method is described directed to multiple thermal or chemical pretensioning. This method is supposedly also suitable for borosilicate glasses; however, there is no suggestion in the entire publication as to how borosilicate glasses should be chemically pretensioned, that is, specifically which composition such borosilicate glasses should have and under what method conditions and with what results. It is only simply asserted that all glasses having a thermal coefficient of expansion .alpha. between 3.0.times.10.sup.-6 K.sup.-1 and 9.5.times.10.sup.31 6 K.sup.-1 and a modulus of elasticity E of between 6.0.times.10.sup.4 N/mm.sup.2 and 9.0.times.10.sup.4 N/mm.sup.2 are suitable (without a solution of this aspect and without a teaching as to how to proceed experimentally). All embodiments relate to the thermal pretensioning of soda-lime glass plates.
The invention described in U.S. Pat. No. 5,599,753 has the object, inter alia, to provide a borosilicate glass which can be chemically tempered. It is presented that glasses, which have no lithium ion content, are therefore not suitable for chemical tempering below the transformation point Tg.
Li.sub.2 O is presented as an optional component of the borosilicate glasses weak in boric acid; however, it is a necessary constituent in the preferred embodiments and, from the disclosure, it can be seen that Li.sub.2 O is regarded as decisive for a glass which can be hardened below the transformation temperature Tg.
Lithium oxide has, as all other alkali oxides, the disadvantage in glass that it increases the thermal expansion and reduces the chemical resistance. Furthermore, Li.sub.2 O reduces the viscosity thereby increasing the separation tendency and the nucleation tendency of the glass whereby turbidity can occur.
Japanese patent publication 4-70262 discloses that borosilicate glasses, which have a very high component for these glasses of alkali oxides (10 to 32 percent by weight R.sub.2 O), can be chemically pretensioned. However, these glasses exhibit a high thermal expansion because of this high alkali content.